I could not find a price for in-game Sextants, but did find a description and picture (attached) in the Barsaive Boxed set. The BBS also has a map marked up to be used with the Sextant.

The "Rod" is just a ruler, marked for "days walking and days riding". The "Astrolabe" is just a weird protractor. I would ether call them "Navigation tools" or "Craftmans Tools: Mapmaking" and assign a price. Maybe 25 silver for a simple, functional copper version.

Time would depend totally upon the level of detail on the map. A quick sketch with only three or 4 landmarks might be knocked out in only a few minutes (once you get your writing kit set up, your ink mixed and your quill sharpened). Large and detailed maps could easily take a time measured in days, weeks, or months. And that would just be the time required to draw it, not the time to explore and measure an area. Exploring the area should require frequent stops to write down observations and distances if you hope to make a map from it. I could see some people adding to their maps in the evenings, while others might just make numerous notes and not attempt actual drawing until the expedition is completed.